Personally, I like the idea of being able to choose from the various armies I have and field an allied force...WoC, Dark Elves and Deamons sounds like a nice mix! although Wood Elves, Dwarves and Brets could work

How had you envisaged the composition rules working for allies? There are a number of possibilities. same rules apply ( min 25% core, max 25% lords etc ) for each 'force', a true mix and match ( probably the most abusable? ), each unit moves up a slot ( core become special, special become rare, you can't take rare ), you can take a maximum of 25% of your army from the ally etc... or you can't have mixed forces in a single battle apart from the last game. i.e.. you essentially have multiple armies to move around ( allowing for people building new armies to fight in the outer rims , while the larger forces battle it out in the centre ). Would there be a limit on how many army books you can choose from?
Another option for allowing people to bring toys is to allow scrolls of binding from Monstrous Arcanam and Pacts from Storm of Magic?